This disclosure is related to gallium nitride based semiconductor transistors.
Gallium nitride (GaN) semiconductor devices, which are III-V or III-nitride type devices, are emerging as an attractive candidate for power semiconductor devices because the GaN devices are capable of carrying large currents and supporting high voltages. Such devices are also able to provide very low on-resistance and fast switching times. A high electron mobility transistor (HEMT) is one type power semiconductor device that can be fabricated based on GaN materials. As used herein, GaN materials that are suitable for transistors can include secondary, tertiary, or quaternary materials, which are based on varying the amounts of the III type material of AlInGaN, Al, In and Ga, from 0 to 1, or AlxInyGa1-x-yN. Further, GaN materials can include various polarities of GaN, such as Ga-polar, N-polar, semi-polar or non-polar. In particular, N-face material may be obtained from N-polar or semi-polar GaN.
A GaN HEMT device can include a III-nitride semiconductor body with at least two III-nitride layers formed thereon. Different materials formed on the body or on a buffer layer causes the layers to have different band gaps. The different materials in the adjacent III-nitride layers also causes polarization, which contributes to a conductive two dimensional electron gas (2DEG) region near the junction of the two layers, specifically in the layer with the narrower band gap. One of the layers through which current is conducted is the channel layer. Herein, the narrower band gap layer in which the current carrying channel, or the 2DEG is located is referred to as the channel layer. The device also includes a gate electrode, a schottky contact and an ohmic source and drain electrodes on either side of the gate. The region between the gate and drain and the gate and source, which allows for current to be conducted through the device, is the access region.
The III-nitride layers that cause polarization typically include a barrier layer of AlGaN adjacent to a layer of GaN to induce the 2DEG, which allows charge to flow through the device. This barrier layer may be doped or undoped. In some cases, doping of the barrier layer may add to channel charge and it may also help in dispersion control. Because of the 2DEG typically existing under the gate at zero gate bias, most III-nitride devices are normally on or depletion mode devices. If the 2DEG is depleted, i.e., removed, below the gate at zero applied gate bias, the device can be an enhancement mode or normally off device.
Enhancement mode or normally off III-nitride type devices are desirable for power devices, because of the added safety they provide. An enhancement mode device requires a positive bias applied at the gate in order to conduct current. Although methods of forming III-nitride enhancement type devices are known, improved methods of depleting the 2DEG from under the gate in the channel layer are desirable.